Problem: Simplify the following expression: $y = \dfrac{n}{5} - \dfrac{n}{9}$
In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $5$ and $9$ $\lcm(5, 9) = 45$ $ y = \dfrac{9}{9} \cdot \dfrac{n}{5} - \dfrac{5}{5} \cdot \dfrac{n}{9} $ $y = \dfrac{9n}{45} - \dfrac{5n}{45}$ $y = \dfrac{9n -5n}{45}$ $y = \dfrac{4n}{45}$